Signal conditioning circuits such as difference amplifiers are often employed in data acquisition applications where isolation is desirable between the potentially much higher voltage of the monitored equipment and the lower voltage of the signal conditioning circuit, for example.
A difference amplifier amplifies the potential difference between its two inputs. Differential signals can be found in many applications, from simple ohmic drops across a sense resistor in a current sensing application to the output of a standard resistive bridge or load cell. Normally the sensed differential voltage is fairly small; usually of the order of microvolts to perhaps hundreds of millivolts.
Usually associated with every differential voltage is a common mode voltage. This common mode voltage can be of an order much, much higher than the differential signal. Each side of the sensed differential signal, when referred to some common potential like zero volts or ground, can be at a voltage potential far above or below the common zero volts or ground. This voltage, which is common to both sides of the sensed differential signal, is called the common mode voltage. This common mode voltage generally contains no useful information about the measurement signal so, ideally, the difference amplifier should amplify only the difference between the signals at its two inputs and ignore or reject the common mode voltage. Depending upon the application, this common mode voltage could be up to several hundreds volts or even higher. It could also be positive or negative with respect to a common point like ground or zero volts.
Additionally in some industries like process control, it is necessary to provide galvanic isolation between the sensed signal and the measuring circuitry. In other words there can be no ac or dc path between the sensed signal which has an associated common mode voltage and the subsequent signal conditioning circuitry which can have a different common mode voltage that is usually zero volts.
To implement an input signal conditioning system capable of meeting all such requirements in the past, a common approach has been to employ a technique called a flying capacitor technique. In this approach a capacitor is switched, either by optically controlled switches, or relays, from across the signal source to across the difference amplifier inputs. By such means the input common mode voltage is removed completely and only the differential input signal is presented to the difference amplifier. This approach can result in a solution which consumes a relatively large area of printed circuit board or printed wire board due to the mechanical relays. Other techniques which have been successful are those that use current transformers and those that are based upon Hall effect current transformers. The disadvantages of the current transformer approach include large physical size and weight, especially as measured current increases, high cost, inability to measure dc currents and voltages and limited accuracy. Disadvantages of the Hall effect approach include low accuracy with an open loop sensor, the need for an isolated power supply to power the isolated side, offsets due to the Hall effect elements and high cost and large physical size for closed loop sensors.
Other approaches that are not based upon a flying capacitor technique are generally based upon a difference amplifier driving an analog-in, analog-out, isolation amplifier. The isolation amplifier is generally transformer-based, employing magnetic coupling to achieve both common mode voltage isolation and signal transmission. Both the difference amplifier and the isolated side of the isolation amplifier are powered from an isolated power supply with a sufficient continuous isolation-voltage rating. Disadvantages of this approach include large physical size, high cost, the need for an isolated power supply to power the isolated side and poor accuracy.
Another approach is to employ an analog to digital converter such as a delta-sigma modulator to digitize an analog input voltage. The digital bit stream from the delta-sigma modulator is then transmitted across an isolation barrier, either an optical barrier—more usual—or a magnetic barrier, and decoded on the far side of the barrier to produce a serial output data stream representing the analog input or, more usually, the transmitted data stream is decoded and converted from the digital domain into the analog domain to produce an analog voltage on the output side of the barrier. Disadvantages of this approach include the need for an isolated power supply to power the isolated side, the need to filter the data stream to extract the sensed signal, high frequency clock and data signals that potentially are a cause of radio frequency interference (RFI), front-end offset drift and gain drift issues over time and temperature, and a relatively expensive low-drift sense resistor.